The present invention relates generally to cellular telephones, and more particularly, to cumulative operational timers and timing computational methods for use in such cellular telephones.
U.S. Pat. No. 4,903,327, entitled "Cellular Telephone With Transmission-On and Radio-On Timers" attempts to address an operational timer problem that exists in the cellular telephone environment. Previous operational timer designs, such as that disclosed in the above-cited patent, have stored nonvolatile memory data at power down time. While this is relatively important thing to do, it can lead to early device failures of typical nonvolatile memory circuits.
An electronically erasable programmable read-only memory (EEPROM), often used as the nonvolatile memory device, only permits about 10,000 write cycles under typical operating conditions. This limit can be further reduced in the demanding mobile telephone environment. For commercial applications of cellular telephony (sales people, police, taxis, etc.), a mobile telephone can have ten or more ON/OFF power cycles per day. A write cycle at each power cycle would lead to an unacceptable device life on the order of 1000 days.
The use of cumulative timers in a mobile telephone is very desirable, in that it permits factory personnel to calculate actual mean time between failure (MTBF) data based upon customer returns. Timer information may also be used to indicate warranty fraud.
Accordingly, it would be an advance in the mobile telephone art to have a cellular telephone design that incorporates a cumulative operational timer and a timing computation method that are not subject to the above deficiencies. It is also an objective of the present invention to provide for an operational timer design and timing computational method that permits factory personnel to calculate mean time between failure (MTBF) data to determine possible warranty fraud.